This disclosure is related to the field of electromagnetic induction well logging. More specifically, the disclosure relates to techniques for characterizing fractures in subsurface formations penetrated by a wellbore that is highly inclined with respect to the fracture plane.
Finding the state of fractures in subsurface formations became important following the advent of what is termed “unconventional production”, or using wellbores that traverse a formation substantially along its bedding plane to cause the wellbore to intersect large numbers of fractures in such formations, such fractures being inclined or perpendicular to the bedding plane of the formations.
Methods known in the art for detecting and characterizing fractures use, for example, borehole imaging instruments that include small (several centimeter) scale electrical resistivity and/or acoustic detectors disposed in pads placed in contact with the wall of a wellbore. These instruments make very shallow (i.e., lateral depth into the formation from the wellbore wall) measurements with respect to the wellbore wall and produce images of features essentially on the borehole wall. A good image from such instruments often requires that the wellbore is in good mechanical condition, i.e., having a smooth, uninterrupted wall free of cave-ins, washouts, etc. The drilling process itself often introduces many very shallow fractures that may be observable on the image to make it difficult for an interpreter to differentiate naturally occurring, greater lateral extent fractures from shallow, induced fractures.
Methods for using much deeper investigating multiaxial (triaxial) induction measurements to detect and characterize fractures have been introduced more recently. These methods may preferentially detect only those fractures that have substantial lateral extent from the wellbore and therefore may provide a differentiation capability that is lacking when using borehole imaging tools. However, multiaxial induction methods known in the art have proven to be most effective under the conditions of a nearly vertical well detecting near vertical fractures, i.e., the fracture plane and the wellbore axis are substantially parallel. Such methods are adequate for exploratory wells have not proven effective for unconventional production wells which are mostly drilled essentially parallel to the bedding plane of the fractured producing formation and thus at high relative angle between the wellbore axis and the fracture plane.
Very thin fractures having large planar extent filled with electrically non-conductive drilling fluid (e.g., oil based drilling mud—“OBM”) may block induced eddy currents from flowing in the formation and could produce significant anomalies in inverted formation parameters compared with those from the same formation without such fractures. The size of the anomaly depends on the formation resistivities (Rh, Rv), the size of the fracture plane, and the relative dip and azimuth between the fracture plane and the layering structure of the formation. The most common fracture system encountered in unconventional productions wellbores is substantially horizontally layered formation having substantially vertical fractures. Therefore, a tri-axial induction well logging instrument may be used to detect and characterize an important part of the large vertical fracture system encountered by a wellbore drilled along the bedding plane of such a formation.
U.S. Pat. No. 6,798,208 B2 issued to Omeragic, U.S. Pat. No. 6,924,646 B2 issued to Omeragic and U.S. Pat. No. 6,937,021 B2 issued to Rosthal describe various methods for using electromagnetic induction measurements to estimate fracture orientation. None of the foregoing patents, however, disclose a method to detect the existence of fracture. All three of the foregoing patents demonstrate that if a large planar fracture is present near the wellbore, the fracture azimuth can be computed from certain electromagnetic induction component measurements oriented perpendicular to the fracture plane. However, such technique may be less valuable without the capability of identifying the existence of the fracture first. The algorithms in the foregoing patents compute an orientation which may also be due to dipping (i.e., non-horizontal) electrically anisotropic formations and have nothing to do with fractures. From a practical point of view, it is useful to have a fracture indicator first than to have a means to compute the fracture azimuth assuming a large fracture exists near the wellbore.
Usually, for resistive fractures in a conductive background formation, the triaxial induction instruments' measurements are relatively insensitive to the fracture aperture. This is because fracture planes having sufficient resistivity contrast with respect to the background formation will block the induced eddy currents in a similar manner regardless of the thickness (or fracture aperture) of the resistive fracture. Therefore, 0.1 inch aperture fracture will cause similar triaxial induction instrument responses as those from a 1 inch aperture fracture. A typical resistive fracture disposed in a conductive background formation condition is a result of OBM drilling through low resistivity fractures shale. Under this condition, using techniques known in the art it may be possible detect the location of fractures and their orientation. However, the measurements do not have sufficient sensitivity to infer the aperture of the fractures accurately.
Under the reverse logging condition, namely conductive fractures within resistive background formations such as water based mud (WBM) logging within high resistivity formations such as carbonates, organic shale, lignite or coal beds, the triaxial induction tool will have sufficient sensitivity to infer the aperture of the fractures. Most of the fractures, natural or induced, in petroleum production applications are nearly vertical. “FRACTURE CHARACTERIZATION USING TRIAXIAL INDUCTION TOOLS”, Peter Wu, et al., paper D, SPWLA 54th Annual Logging Symposium, New Orleans, La. Jun. 22-26, 2013, discloses a method for obtaining estimation of an effective fracture aperture for a near vertical fracture system encountered near the wellbore using triaxial induction instrument measurements. The foregoing described method exploits the sensitive components of the measured apparent conductivity tensor or transimpedance coupling voltage measurements and inverts for effective fracture aperture using a simple model of uniform anisotropic formation background with a large vertical fracture parameterized by an arbitrary aperture width.